Screw galvanized and Dacro coated features
Of all the coatings, zinc is the cheapestcompared with other electroplating, and zinc corrosion resistance is far betterthan steel in most cases. Zinc coating can protect the metal itself fromdiscontinuous holes, cracks and cut edges. So the general screw or easy to beeroded by water vapor under the screw is mostly galvanized, but the screw isnot flat smooth, so the thickness of galvanized layer in each part is hard toeven. The thickest head, then the front of the thread hill, the thinnest threadvalley. In order to cooperate with the mass production of screws, reduce theproduction cost and increase the output, it is often used to galvanize thebucket. This method is to make the electric current flow from the electrode ofthe bucket into the wire of the bucket, and the wire is in contact with thescrew and conduct electricity, while stirring the screw to make the screw andthe electrode evenly contact. However, intermittent current is difficult toform a uniform current density, so galvanized thickness is also different.
The use of galvanizing acid electroplatingsolution and alkaline electroplating solution two. The acid electroplating bathis mainly composed of zinc sulfate. Low price and high efficiency,electroplating speed and convenient management than alkaline electroplatingsolution is not easy to hydrogen brittleness. But the crystallization iscoarse, luster is not good, not uniform. The alkaline electroplating solutionis mainly composed of zinc cyanide. Although the price is relatively high, butthe plating ability is good, as long as the management is appropriate, you canget a shiny coating. If further chromate treatment is performed. A brightercoating and better corrosion resistance can be obtained.
Hot dip galvanizing is also used for thesurface corrosion protection of screws, bolts and nuts. Hot dip galvanizedscrews have better corrosion resistance than electrically galvanized screws.However, high cost and high strength structural bolts are not suitable for hotdip galvanizing, because the surface of the coating is easy to form a hardbrittle at high temperature or high load conditions may fracture. There is alinear relationship between the durability of galvanized coating in corrosiveenvironment and its thickness, that is, the thicker the coating, the longer thelife of galvanized coating. However, the corrosion resistance of galvanizedcoating is poor in industrial zones containing sulfide. At the same time, alarge number of self-colored corrosive products will be produced in Marine andtropical climate.
The highest corrosion rate is in the areawith the highest SO2 and chloride (sea salt particles) content in theatmosphere and the highest relative humidity. Atmospheric relative humidity isconsidered as the main factor causing corrosion, and atmospheric pollutants,especially SO2 and chloride, are the main factors promoting corrosion.Galvanized screw in the production, storage, transportation process of itsgalvanized layer may appear white, dark, black and other phenomena, mainlygalvanized layer and corrosive media and atmospheric water, oxygen, carbondioxide in contact with the result of corrosion.
Currently on the market is a metal surfacetreatment called Dacro, suitable for a variety of machine screw, bolts and nutsand other fasteners. The treatment method is to dip the degreased and derustingscrew nut into the anti-rust agent tank of Dag rust or to use the sprayingmethod to form the protective film of zinc and chromium compound on the metalsurface, and then to dry and cool and then to be treated again. The thicknessof the film is about 5~8 (silver gray) according to the data shows that thefilm has good anti-rust and anti-corrosion characteristics. The researchersused what they called a zinc-chromate coating treatment on high-strength steelbolts, and two years later found it worked.
It was found that adding aluminum to zinccould improve the corrosion resistance of the coating. When the aluminumcontent is 30%, zinc-aluminum alloy has the best corrosion resistance, so thechemical stability and corrosion resistance of this kind of coating is betterthan zinc plating alone.
Hongsheng screw production of various whitezinc plated screws, color zinc plated screws, black zinc plated screws, platedDacro screw.
